Deeply hardenable liner member for grinding mill



y 1966 w. R. HUBER ETAL 3,250,610

DEEPLY HARDENABLE LINER MEMBER FOR GRINDING MILL Filed Dec. 51, 1962 2 Sheets-Sheet l INVENTORS WILL/AM R. HUBER and GEORGE E. r/s/lvA/ y A from ey May '10, 1966 Filed Dec. 51, 1962 w. R. HUBER ETAL 3,250,610

DEEPLY HARDENABLE LINER MEMBER FOR GRINDING MILL 2 Sheets-Sheet 2 l l I l l l I l l l l l I 0.02 0.04 0.06 0.08 0./0 0./2 0./4 0./6 0J8 0.20 0.22 0.24 0.26

MOLYDE/VUM, per cent l l l l l l l l l I Q Q Q Q Q Q Q Q Q Q Q Q a s g a s a Q a. a 2 m K m o v a; w //V|/E/V7'0R.S COOL/N6 RATE, degrees Fper hour WLL/AM HUBER GEORGE F T/S/IVA/ y Attorney United States Patent 3,250,610 DEEPLY HARDENABLE LINER MEMBER FOR GRINDING MILL William R. Huber, Johnstown, and George F. Tisinai,

Monroeville, Pa., assignors to United States Steel Corporation, a corporation of Delaware Filed Dec. 31, 1962, Ser. No. 248,500 7 Claims. (Cl. 75-126) This invention relates to deep-hardening, low-alloy steel compositions particularly suited for use as lining elements in ball mills, rod mills and the like.

Many industrial processes involve comminuting relatively hard materials, e.g. ore, coal and cement-clinker. This is commonly done by crushing the material to an intermediate size and then tumbling the crushed material together with grinding-balls or rods of hardened steel in a cylindrically shaped vessel. The latter, disposed in a substantially horizontal position, is rotated about its longitudinal axis and is provided with a replaceable, abrasion-resistant lining which serves as the anvil element in the grinding action. To facilitate replacement, sectional linings bolted to the shell of the vessel are used.

Replaceable linings consisting of rows of plate-like components with separate longitudinally extending liftbars projecting inwardly between the adjacent rows to facilitate the tumbling action have been used. In a more efiicient arrangement, however, the plate and lift members are incorporated in a single element having the general cross'section indicated in FIGURE 1; such lining elements are typically about 18 inches wide, about 4 inches thick at the edges, about 7 inches thick at the center of the section. The elements are made to a length which is a convenient sub-multiple of vessel length, an average element being about 3 feet long. The economies of such heavy single-component elements are realized only when high hardness is achieved throughout the section thereof. And, in this regard, experience has shown that hardness at the center of the section must be at least 50 on the Rockwell C sca'le (50R Heretofore,'only the alloyrich cast-iron and the high-alloy steels have afiorded the requisite degree of hardenability; these materials, however are very expensive and difficult to work. In addition, the cast materials are relatively brittle and tend to spall during service.

The less massive elements of the two-component lining system have been produced from low-cost, low-alloy steels of the following analysis:

C, 0.60-0.75%; Mn, 1.401.70%; Si, OAS-0.65%; Cr, 0.70-0.90%; balance, iron plus residuals of other ele ments. However, in sections more than about 4 /2 inches thick the maximum center hardness reliably attainable is about 40R A slightly deeper hardening can be achieved by quenching from above 1700 F. but the incidence of failure by distortion, quench-cracking, and the like increases rapidly when temperatures above about 1550 F. 'are used. Moreover, the presently available low-alloy compositions are extremely sensitive to the cooling rates obtaining during quenching, with the result that slight variations in the circulation of the quenching medium produce soft-spots. As a consequence of these deficiencies the presently available low-alloy steels, although offering a considerable advantage price-wise, cannot be applied in the manufacture of single-component lining elements.

Accordingly, it is an object of the present invention to 3,250,510 Patented May 10, 1966 acter which is relatively insensitive to variation in quenching rate.

The accomplishment of these and other objects will be made clear in the following specification when read in conjunction with the appended drawings wherein,

FIGURE 1 is a cross-section through a typical singlecomponent lining element used in ball mills and the like;

FIGURE 2 is a graph showing the effect of variation in molybdenum content on the hardenability of the lowalloy steel compositions with which the present invention is concerned;

FIGURE 3 is a graph showing the relationship of the cooling rate obtaining during quenching to the hardness achieved thereby in the low-alloy steel compositions provided by the present invention and the low-alloy compositions available heretofore;

FIGURE 4 is a hardness survey of a half-section of a lining element made in accordance with the practices of the present invention and illustrates the depth and uniformity of hardening achieved thereby;

FIGURE 5 is a hardness survey of a similar half-section but made of the shallow-hardening low-alloy steel available heretofore, the shaded area outlining the area of inadequate hardening.

We have discovered that a limited amount of Mo, between about 0.10 and 0.25%, in combination with C, 0.55 to 0.75%; Mn, 1.60 to 1.90%, Si, 0.50 to 0.90% and Cr, 0.50 to 0.90%, provides a steel which behaves in an entirely unexpected manner, i.e. these steels, when quenched from above about 1550 F.,. are, in view of their relatively low-alloy content, exceptionally deep-hardening and exhibit this behavior over an unusually wide range of cooling rates. Their unique behavior, which to the best of our knowledge we have been the first to discover, is best shown by the curves of FIGURE 2 wherein curve I represents the actual hardenabilities obtained in our tests of a series of above steels containing varying amounts of Mo; while curves II and III represent the expected hardenabilities of the compositions are predicted from current metallurgical knowledge of the effects of the alloying elements present. Curve II is specific to hardenabilities as calculated using the AISI procedures and multiplying factors, see'ASM Handbook, 1948 edition, page'500; curve III, to the hardenabilities as calculated per Whittenberger et al., see A.I.M.E. Trans. 1956, vol. 206, pages 1008-1016. The hardenabilities, i.e. the depth of hardening, have been expressed in terms of ideal diameter (D The latter has been defined as the diameter of a round of a steel which, when given an ideal quench will quench at its center to a desired microstructure. A structure consisting of 50% martensite is taken as the reference structure. Since the relationship between the cooling rate in the ideal quench and the rates in various actual quenching practices are well established, see Making, Shaping and Treating of Steel, 7th edition, page 806, any set of hardening test data specific to a given steel is readily translatable into the fundamental terms of ideal diameter. As indicated above the ideal diameter to be expected of any steel is calculated directly from the alloy content thereof per AISI or Whittenberger et al. For steel containing carbon in the range 0.5 to 0.8, the hardness of a 50% martensitic structure is commonly taken as about SOR The chemical analysis of individual steels of the test series are given in Table I; the ideal diameters" of each,

based on measurement and as calculated, are also tabu- TABLE 1 Composition, percent Ideal Diameters Steel Identifil Meas- Caleu- Calcucation Mn Si Or Mo ured by lated lated This A181 1 Whitten- Work berger 1. S1 0. 61 0. 77 0. 00 12. 2 9. 2 10.9 1. 77 0.58 0. 76 0. 05 14. 0 10. 3 12.3 1. 79 0. 66 0. 82 0. 12 20. 3 13. 6 l3. 2 1. 74 0. 58 0. 83 0. 13 23. 5 12. 7 14. 0 l. 75 0. 68 0. 81 0. 14 22. 8 14. 2 l3. 2 1. 76 0. 66 0. 80 0. 14 22.8 13. 9 l3. 0 1. 78 0. 66 0. 82 0. 16 26. 0 14. 3 14. 2 1. 75 0. 63 0. 76 0. 20 24. 8 14. 4 l4. 1 1. 75 0. 60 0. 75 0. 26 19. 8 15. 8 14. 8 1. 62 0.80 0. 92 0. 17 20. 0 14. 2

1 Using data for t. grain size between 5 and 6.

As shown in FIGURE 2, the effect on hardenability of additions of molybdenum up to about 0.05% in steels containing C, Mn, Si and Cr within the forementioned ranges is substantially that predicated by current theory, but with additions of more than about 0.08% M0 the actual hardenability becomes increasingly greater than the theoretical reaching a maximum in this respect at about 0.16% Mo. While the amount of deviation declines with further addition of vMo, return to expected hardenability does not occur until about 0.30% Mo has been added. The magnitude of the deviation is remariv able in that an improvement of at least 50% is afforded over the range of about 0.10 to 0.25% Mo, while almost double the expected hardenability obtains in the range of about 0.14 to 0.19%. In summary, the discoveries of the present invention in the sections involved in grinding mill liner elements and under the conditions of practical heat treatments, provide low-alloy steels hardenable in contrast as shown by curve A, typical of the low-alloy steels heretofore available, a quenching rate of at least 3100 F. per hour is required for similar results. The relative insensitivity of the new steels to quenching rate greatly mitigates the difficulties encountered in oil quenching and expands the quenching procedures useable, eg the new compositions can be hardened by forced-aircooling. Hot Working the steels, as by forging or rolling, is also facilitated.

Achievement of the foregoing results requires the particular combination of alloying elements present in the disclosed ranges. That is, as already noted from curve I of FIGURE 2, the results are lost by overalloying with Mo; a similar loss occurs when the steels are overalloyed with carbon. The latter is illustrated by the following examples.

TABLE II Compositions and ideal diameters of steels with more In addition, we have found that while the Mn and Si can be varied Within aforestated ranges, the total of Mn and Si should not be less than 2.30%. As to Cr, at least 0.50% of this element must be present, while Cr in excess of about 0.90% is detrimental to results by overalloying the compositions as Well as adding to the costs thereof.

It is apparent from the foregoing discussion that the steels of the present invention are ideally suited to the manufacture of grinding mill liners and in similar applications wherein high hardness throughout a heavy section is essential. Subject to the limitations noted above, any composition encompassed by the analysis tabulated below can be used; however, optimum results are associated with the more limited ranges listed in the last column of the table.

Percent Preferred Range 0. -0. 75 0. -0. 1. 60-1. 90 1. 70-1. 0. 50-0. 0 60-0. 80 0. 50-0. 90 0. 60-0. 80 0. 10-0. 25 0. 11-0 20 Bel. B

*Iron including the usual residual amounts of the other elements, e.g. P, S, N2, etc. encountered in steelmaking operations.

Although any of the above compositions can be used in the manufacture of single-component lining elements where oil quenching is to be used, the Mo content is prefferably held between 0.11 and 0.15%, i.e. we prefer a steel melted to the following aim:

C Mn Si Cr M0 whereas if quenching by forced-ai-r-cooling is to be used, the M0 is preferably held between 0.16 and 0.20, i.e. the steel is melted to 0.18% Mo aim. The steel, after hot working by forging or rolling to sectional shape, may be given a spheriodizing anneal to facilitate dressing of the ends and edges and other machining operations incident to fabrication of such lining elements. Upon completion of such operations, the elements are austenitized at about 1550 F.; higher temperatures are unnecessary and undesirable, their use tending toward the retention of austenite during quenching. While, as noted above, forced-air-quenching can be practiced, we prefer to oil quench from austenitizing temperature to below about 700 F. at the center of section, and thentemper at about 400 F. The above practices produce elements of the size and shape described in connection with FIGURE 1, hardened throughout to at least 50R as illustrated by the hardness traverses recorded in FIGURE 4.

The improvements achived by our invention are evident upon comparison of FIGURES 4 and 5. The latter records the hardness traverses made on a similar element heat treated in the same manner as the element of FIG- URE 4, but forged from the low alloy steel composition previously used. While high surface hardness is achieved with such compositions, the hardness drops rapidly and in the shaded area is inadequate for intended service of the part. As evident from the hardness measurements of FIGURE 4, the microstructure at the center of the low-alloy product of the present invention comprises at least 50% martensite, thus providing single-component lining elements having a service life equal to that of elements made from the high-alloy irons and steels but at a cost substantially below the cost of the latter.

While we have shown and described certain preferred embodiments of our invention, it is apparent that other modifications may arise. Therefore, We do not wish to be limited to the disclosure set forth but only by the scope of the appended claims.

We claim:

1. Low-alloy steel characterized byhigh hardenability in thick sections when quenched from austenitizing temperature at rates as low as 900 F. per hour, said steel consisting essentially of 0.55 to 0.75% carbon, 1.60 to 1.90% manganese, 0.50 to 0.90% silicon, 0.50 to 0.90% chromium, 0.10 to 0.25% molybdenum with the balance, iron and residual amounts of other elements, the total of manganese and silicon being at least 2.30%.

2. Low-alloy steel characterized by high hardenability in thick sections when quenched from austenitizing temperatures at rates as low as 900 F. per hour, said steel consisting essentially of 0.60 to 0.70% carbon, 1.70 to 1.80% manganese, 0.60 to 0.80% silicon, 0.60 to 0.80% chromium, 0.11 to 0.20% molybdenum, with the balance, iron and residual amounts of other elements, the total of manganese and silicon being at least 2.30%.

3. Low-alloy steel characterized by high hardenability in thick sections when forced-air-quenched from about 1550 F, said steel consisting of 0.60 to 0.70% carbon, 1.70 to 1.80% manganese, 0.60 to 0.80% silicon, 0.60 to 0.80% chromium, 0.16 to 0.20% molybdenum with the balance, iron and residual amounts of other elements, the total of manganese and silicon being at least 2.30%.

4. Low-alloy steel characterized by high hardenability in thick sections when oil quenched from about 1550 F., said steel consisting essentially of 0.60 to 0.70% carbon, 1.70 to 1.80% manganese, 0.60 to 0.80% silicon, 0.60 to 0.80% chromium, 0.11 to 0.15% molybdenum with the balance, iron and residual amounts of other elements, the total of manganese and silicon being at least 2.30%.

5. A hot-worked and hardened low-alloy steel grinding mill liner element characterized by a hardness of at least 50 Rockwell C throughout the thickness thereof consisting essentially of 0.55 to 0.75% carbon, 1.60 to 1.90% manganese, 0.50 to 0.90% silicon, 0:50 to 0.90% chromium, 0.10 to 0.25 molybdenum with the balance,

iron and residual amounts of other elements, the total of manganese and silicon being at least 2.30%.

6. A hot-worked and hardened low-alloy steel grinding mill liner element characterized by a hardness of at least Rockwell C throughout the thickness thereof consisting essentially of 0.60 to 0.70% carbon, 1.70 to 1.80% manganese, 0.60 to 0.80% silicon, 0.60 to 0.80% chromium, 0.11 to 0.20% molybdenum with the balance, iron and residual amounts of other elements, the total of manganese and silicon being at least 2.30%.

7. A method for producing grinding mill liner elements comprising making a steelconsisting essentially of 0.55 to 0.75% carbon, 1.60 to 1.90% manganese, 0.50 to 0.90% silicon, 0.50 to 0.90% chromium, 0.10 to 0.25% molybdenum with the balance, iron and residual amounts of other elements, the total of manganese and silicon being at least 2.30%, hot working said steel to a desired liner element shape, austenitizing said shape at about 155 0 F. and then quenching said shape firom tau-stenitizing temperature to at least 700 F. at a quenching rate of at least 900 F./hour thereby hardening said shape to at least 50 Rockwell C throughout the thickness of said shape.

References Cited by the Examiner UNITED STATES PATENTS 6/1924 Corning et al. -126 7/1929 Hamilton et al. 75-126 DAVID L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner. 

1. LOW-ALLOY STEEL CHARACTERIZED Y HIGH HARDENABILITY IN THICK SECTIONS WHEN QUENCHED FROM AUSTNITIZING TEMPERATURE AT RATES AS LOW AS 900*F. PER HOUR, SAID STEEL CONSISTING ESSENTIALLY OF 0.55 TO 0.75% CARBON , 1.60 TO 1.90% MANGANESE, 0.50 TO 0.90% SILICON, 0.50 TO 0.90% CHROMIUM, 0.10 TO 0.25% MOLYBDENUM WITH THE BALANCE, IRON AND RESIDUAL AMOUNTS OF OTHER ELEMENTS, THE TOTAL OF MANGANESE AND SILICON BEING AT LEAST 2.30%. 